1. Field of the Invention
The invention relates to a mixing device that mixes exhaust gas that flows through an exhaust pipe of an internal combustion engine, with an additive sprayed into the exhaust pipe by an adding valve.
2. Description of Related Art
An exhaust gas control apparatus that purifies oxides of nitrogen (NOx) in exhaust gas by reducing NOx to water and nitrogen is provided in an internal combustion engine. In such an exhaust gas control apparatus, an additive that is a urea aqueous solution is sprayed from an adding valve into an exhaust pipe, and this additive is supplied to an exhaust gas control catalyst arranged on an exhaust gas downstream side (i.e., downstream with respect to the direction in which exhaust gas flows) of the adding valve. Also, a mixing device that is arranged between the adding valve and the exhaust gas control catalyst in the exhaust pipe is provided in the exhaust gas control apparatus (see Japanese Patent Application Publication No. 2008-280999 (JP 2008-280999 A)).
The mixing device described in JP 2008-280999 A is provided with a cylindrical portion that has a cylindrical shape, a plate for mixing exhaust gas and additive that have flowed into the cylindrical portion, and an annular flange that extends radially outward from an outer peripheral surface of the cylindrical portion. A plurality of supporting portions are provided on an inner peripheral edge of this flange, and the flange is attached to the cylindrical portion via these supporting portions. The mixing device is provided in the exhaust pipe, with the flange sandwiched between the exhaust pipe on the upstream side and the exhaust pipe on the downstream side.
FIG. 7 is an exploded perspective view of the mixing device. As shown in FIG. 7, supporting portions 100 are formed by extending pieces that extend inward from an inner peripheral edge of a flange 110 that have been bent. The direction in which the supporting portions 100 extend matches an axial direction of a cylindrical portion 120. When the cylindrical portion 120 is press-fit into the flange 110 when attaching the flange 110 to the cylindrical portion 120, the supporting portions 100 are brought into surface contact with the outer peripheral surface of the cylindrical portion 120. In this state, the supporting portions 100 are welded to the cylindrical portion 120. The axial direction of the cylindrical portion 120 matches a direction orthogonal to a plane of the flange 110.
In order to ensure good airtightness at a connecting portion between the exhaust pipe on the upstream side and the exhaust pipe on the downstream side, it is preferable to increase the flatness of the flange 110. However, the flange 110 may become distorted as a result of attaching the flange 110 to the cylindrical portion 120, so that the flatness of the flange 110 may decrease.
That is, when the cylindrical portion 120 is press-fit into the flange 110, the supporting portions 100 deform, with tip ends of the supporting portions 100 becoming displaced to the radial outside of the cylindrical portion 120, and try to pivot with portions that are supported by the flange 110 as the fulcrums, as shown in FIG. 8. At this time, the supporting portions 100 may deform such that areas near portions of the flange 110 to which the supporting portions 100 are connected are partially pushed outward due to force from the pivoting of the supporting portions 100 (also referred to as “deformation force P”), as shown by the chain double-dashed lines in FIG. 8. This partial deformation (i.e., deformation in parts) of the flange 110 leads to a decrease in the flatness of the flange 110.
When attaching the mixing device to the exhaust pipe with the flange 110 that has such a low flatness, it becomes necessary to increase the rigidity of the flange by increasing the thickness of the flange so that such deformation will not occur, or provide a strong sealing mechanism, in order to ensure the airtightness at the connecting portion between the exhaust pipe on the upstream side and the exhaust pipe on the downstream side.